Use declarative version specification for Ubuntu GitHub Actions runners

[d33bs]

Description

This PR addresses #197 by setting a specific version for Ubuntu runners through GitHub Actions. It follows thoughts from #195 where we experienced surprising issues after macos-latest began referencing versions which aren't compatible with Python versions we remain compatible with here.

Thank you for any thoughts or feedback you may have!

Closes #197

What is the nature of your change?

• [ ] Bug fix (fixes an issue).
• [x] Enhancement (adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] This change requires a documentation update.

Checklist

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• [x] I have performed a self-review of my own code.
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• [ ] I have made corresponding changes to the documentation.
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• [x] New and existing unit tests pass locally with my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
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[d33bs]

Thanks @kenibrewer ! Merging this in.